The invention is in the field of well logging, in which measurements taken in one or more boreholes by one or more tools drawn therethrough are used in the search for valuable underground resources, such as oil and gas, and concerns finding depth zones consistent with the logged naturally occurring bedding of geological formations and making use of such depth zones. Zoning in accordance with the invention involves finding subsurface depth zones within each of which a number of logs have a respective selected mutual consistency.
It has been proposed to rectangularize or zone individual logs by identifying transitions in an individual log which are believed to be consistent with transitions between beds of the logged geological formations. See, e.g., U.S. Pat. application Ser. Nos. 019,917 now Pat. No. 4,314,338; 019,918 now Pat. No. 4,340,934; 019,925 now Pat. No. 4,314,339 and 019,926 now Pat. No. 4,313,164 filed on Mar. 12, 1979 and assigned to the assignee of this application. Also see Southwick, S. H. et al, Digital Computer Programming For Automatic Analysis Of Well Logs, Paper No. SPE711, to be presented at the 38th Annual Fall Meeting of the Society of Petroleum Engineers of AIME in New Orleans, October 6-9, 1963; Testerman, J. D., A Statistical Reservoir-Zonation Technique, J.P.T. August, 1962, pp. 889-893; and Kulinkovich, A. E. et al, The Location Of The Layer Boundaries And The Differentiation Of Sandstones According To Electrical Logging Data With The Use Of Digital Computers, Prikl. Geofiz. No. 39, pp. 107-113, 1964 (in Russian, with abstract in English attached).
As opposed to typical prior art proposals which emphasize changes in a single log and/or individual log measurements taken at individual depth levels, the invention relies on emphasis on the degree of consistency not only between the measurements of a single log at the depth levels within a given depth zone but also on the consistency between such measurements for several logs for the same zone. By way of an example, if density is the subsurface parameter of interest, a typical prior art emphasis would be in ensuring, to the extent possible, that the density at a given depth level is exactly 2.60 gm/cm.sup.3. This may indeed be important if the formation in the immediate neighborhood of the particular depth level was so isotropic as to share the same exact density. In contrast, the invention herein makes use of the realization that the more likely situation is that there is some random anisotropy and that it is more important to determine that the average density of, say, a four foot depth zone of the formation is 2.61 gm/cm.sup.3.
In accordance with an embodiment of the invention, a fundamental assumption is made that the logged geological formation is made up of layers and that the logged parameters do not change significantly within a given layer. With this in mind, the described embodiment identifies and makes use of depth zones within each of which a given set of logs have a respective selected mutual consistency.
In a particular embodiment, a number of well logs are generated for a common subsurface depth interval. This interval may be the depth of a logged borehole, a portion thereof, or a portion or all of additional boreholes which traverse the same or substantially similar formations. The logs preferably have relatively good vertical resolution: for example, they can be the porosity logs characterized by a vertical resolution of, say, two feet, as opposed to, say, basic resistivity logs which can have a vertical resolution of, say, in excess of four or five feet. For convenience, the preferably higher vertical resolution logs used in the zoning procedure are referred to as "active" logs, and the zones found on the basis thereof are used later to rectangularize "passive" logs, but it should be understood that some or all of the so rectangularized passive logs may have been used earlier as active logs.
Mutually depth-matched portions of the active logs are combined with each other to find subsurface depth zones within each of which the logs have a respective selected mutual consistency. A premise of this aspect of the invention is that a log measurement at a given depth level is consistent with a given zone if it fits within a permissible uncertainty tolerance with all portions of several active logs for the depth levels within the zone. For a given tool, the permissible uncertainty can be considered to be the combination of the probable standard deviation of the actual measurements taken thereby and a chosen acceptable lithological variation. The probable standard deviation can be found in laboratory tests or it can be arbitrarily chosen, and the acceptable geological variation can be arbitrarily chosen or it can be based on some independently derived knowledge of the geology of the logged subsurface formations.
In one particular example, the process starts with a provisional zone consisting of a single depth level and provisionally expands it by successively adding thereto adjacent depth levels for as long as the log portions within the so expanded zone retain said selected mutual consistency. For example, either or both of the depth levels immediately below and immediately above the provisional zone are provisionally added thereto depending on the relative fit thereto, and either or both or none are kept in the so expanded provisional zone depending on whether the depth levels in the expanded zone retain the respective selected mutual consistency. After the process is repeated starting with each successive depth level in the logged depth interval, the result is that each such depth level is associated with a respective provisional zone which can consist of one or more depth levels and which can have the same depth limits as the zone associated with one or more different depth levels. Following the elimination of zones which are not recognized by enough of their constituent depth levels (e.g., when less than half of the depth levels within a zone are associated with the same upper and lower zone depth levels, the remainder still typically comprises some zones which are completely included within other zones of greater depth extent and some zones which are mismatched in that two zones overlap or two depth-adjacent zones are separated from each other by a gap. Sequences of such mismatched zones are located and a test is run to determine which, if any, subset of the zones within such a sequence best covers the depth span of the sequence, and only the zones within such a subset are kept. Any remaining overlaps and any gaps which are within a selected gap limit are resolved by dividing the depth levels within an overlap or such a gap between the adjacent zones through the use of a procedure which tends to ensure best fit between the so divided depth levels and the zones into which they are included. After this, the remaining zones either match up, i.e., one zone starts where another ends, or adjacent zones are separated by a gap greater than the gap limit. An attempt can be made to reconsider the large gaps by relaxing the selected mutual consistency standards (i.e., increasing the permissible uncertainty) and attempting to zone the depth interval of each large gap in accordance with the procedure discussed. The end result for the entire logged subsurface depth interval is a sequence of matched zones the vertical extent of each of which is typically several depth levels but could be a single depth level.
The active and any passive logs for the common depth interval can be converted to rectangularized logs on the basis of the so found zones such that each log has a respective constant measurement level within each respective zone. In one example, this rectangularization can rely on averaging the relevant log measurements within the relevant zones. If the log has only one maximum or minimum situated near the center of the zone, then the average can be boosted toward the peak value in accordance with a procedure which is believed to reflect the geological and other influences on log measurements in such zones. When the active and/or passive logs show rapid and large variations in a relatively small depth interval, i.e., when the average zone length is approximately the same as the vertical resolution of the relevant log, this typically results in a sequence of one-depth-level zones which exhibit monotonic changes and can be selectively deconvolved into a lesser number of acceptable zones. For example, a sequence of one-level zones can be eliminated by assigning the upper half of its depth levels to the zone above and the remainder to the zone below the sequence.
A tangible record of selected parameters of the so found zones is produced; as nonlimiting examples: by producing such a record of the rectangularized active and/or passive logs, by producing one or more crossplots in which a crossplot point is determined by zone parameters rather than parameters for individual depth levels of two or more logs, such crossplot points including, optionally, an analog visual indication of the depth extent of the zones determining the particular point; by plotting zones in several different ways; and/or by storing selected zone parameters.
Rectangularization of logs based on zones found in accordance with the invention tends to eliminate noise components of the relevant logs and thereby clarify the significance of the log measurements. Crossplots produced from zoned logs tend to have more useful contents and less noise than crossplots based on individual depth levels, and tend to facilitate identifying trends of the relevant subsurface formations. Procedures using logs to find other subsurface parameters, which are not susceptible to being logged directly, tend to be speeded up and simplified when they use zones rather than individual depth levels. As another nonlimiting example, zones found in accordance with the invention assist in making additional subsurface measurements, such as in the introduction of stationary measuring devices, for example induced spectroscopy tools, at the optimal location for such tools, for example the approximate center of a zone. Still in addition, the use of active logs of good vertical response to zone a subsurface depth interval makes it possible to zone and thereby sharpen the vertical response of logs from tools which inherently have relatively poor vertical response.